Ranked in these QuestionsQuestion Ranking

Pros

Pro

Will probably be enough for a sizable chunk of users

Since it's the built-in Mac email client and quite complete feature-wise, most users will not really need any other third-party applications out there. Since Mail.app should be enough for their needs.

Pro

Can be easily extended

If the basic features inside Mail.app are not enough, it can easily be extended through plugins to add new features.

Pro

Easily annotate pictures or sign documents inside the client itself

Mail.app has powerful markup tools which let users add annotations to images or even sign documents they got through email without ever leaving the client.

Cons

Con

No option to snooze emails

You can not snooze emails to view later. This makes it hard to keep the inbox clean at all times since you have to read every email and choose to delete or keep them the moment they arrive to do so.

Con

Poor performance for some users on El Capitan

Con

Missing lots of advanced features

Out of the box, Mail.app may not have as many features as some other options. This can be mitigated with plugins though. But that is not a perfect solution since third-party plugins may not be very reliable in the long run.